The present invention relates to the regeneration of nitrogen oxide storage catalysts which are used to remove nitrogen oxides from the exhaust gas from lean-burn engines.
To reduce the fuel consumption of petrol engines, what are known as lean-burn engines which are operated with lean air/fuel mixes in the part-load range have been developed. A lean air/fuel mix contains an oxygen concentration which is higher than necessary for complete combustion of the fuel. In this case, the corresponding exhaust gas contains the oxidizing components oxygen (O2), nitrogen oxides (NOx) in excess compared to the reducing exhaust-gas components carbon monoxide (CO), hydrogen (H2) and hydrocarbons (HC). Lean exhaust gas usually contains 3 to 15% by volume of oxygen. However, at full load, lean-burn engines are also operated with stoichiometric or substoichiometric, i.e. rich, air/fuel mixtures.
By contrast, diesel engines generally operate under conditions with highly superstoichiometric air/fuel mixes. Only in recent years have diesel engines which can also be operated with rich air/fuel mixes for a short period of time been developed. Diesel engines, in particular those with the option of rich operating phases, are also encompassed by the term lean-burn engines in the context of the present invention.
What is known as the air/fuel ratio lambda (λ) is used to characterize the air/fuel mix. This represents the air/fuel ratio standardized to stoichiometric conditions. An air/fuel mix with a stoichiometric composition has an air/fuel ratio of 1. Air/fuel ratios of greater than 1 indicate a lean air/fuel mix, whereas air/fuel ratios of below 1 indicate a rich air/fuel mix. The exhaust gas which leaves the engine has the same air/fuel ratio as the air/fuel mix with which the engine is operated.
On account of the high oxygen content of the exhaust gas from lean-burn engines, the nitrogen oxides contained therein cannot be continuously reduced to form nitrogen, with simultaneous oxidation of hydrocarbons and carbon monoxide, with the aid of three-way catalysts as used in spark-ignition engines operated under stoichiometric conditions. Therefore, what are known as nitrogen oxide storage catalysts, which store the nitrogen oxides contained in the lean exhaust gas in the form of nitrates, have been developed for the purpose of removing the nitrogen oxides from the exhaust gas from these engines.
The operation of nitrogen oxide storage catalysts is described extensively in SAE document SAE 950809. According to this, nitrogen oxide storage catalysts consist of a catalyst material, which has generally been applied in the form of a coating to an inert honeycomb carrier made from ceramic or metal, referred to as a carrier. The catalyst material contains a nitrogen oxide storage material and a catalytically active component. The nitrogen oxide storage material in turn consists of the actual nitrogen oxide storage component, which has been deposited in highly disperse form on a support material. The storage components used are predominantly the basic oxides of the alkali metals, the alkaline-earth metals and the rare earths, but in particular barium oxide, which react with nitrogen dioxide to form the corresponding nitrates.
The catalytically active components used are usually the precious metals from the platinum group, which are generally deposited together with the storage component on the support material. The support material used is predominantly active alumina with a high surface area. However, the catalytically active components may also be applied to a separate support material, which can likewise consist of active alumina.
The role of the catalytically active components is to convert carbon monoxide and hydrocarbons in the lean exhaust gas into carbon dioxide and water. Moreover, they are intended to oxidize the nitrogen monoxide present in the exhaust gas to form nitrogen dioxide, so that it forms nitrates with the basic storage material (storage phase or lean-burn mode), since the nitrogen oxides present in the exhaust gas from lean-burn engines, depending on the operating conditions of the engine, are made up of 65 to 95% by volume of nitrogen monoxide, which does not react with the storage components to form nitrates.
As the accumulation of nitrogen oxides in the storage material increases, the storage capacity of the material decreases. The storage material has to be regenerated from time to time. For this purpose, the engine is briefly operated with air/fuel mixes with a stoichiometric or rich composition (during what is known as the regeneration phase or rich-burn mode). Under the reducing conditions in the rich exhaust gas, the stored nitrates are decomposed to form nitrogen oxides NOx, and reduced to form nitrogen together with water and carbon dioxide. The carbon monoxide, hydrogen and hydrocarbons present in the exhaust gas serve here as reducing agents. The reduction is exothermic and increases the bed temperature of the catalyst by approximately 30 to 50° C. compared to the exhaust gas temperature prior to entry into the catalyst.
During the storage phase or lean-burn mode, the air/fuel ratio is between approximately 1.3 and 5, depending on the type of engine. During the brief regeneration phase or rich-burn mode, the air/fuel ratio is lowered to between 0.7 and 0.95.
When the nitrogen oxide storage catalyst is operating, the storage phase and regeneration phase alternate at regular intervals. The sequence of storage phase and regeneration phase is also referred to hereinbelow as purification cycle. The duration of the storage phase depends on the level of nitrogen oxide emission of the engine and on the storage capacity of the catalyst. In the case of catalysts having a high storage capacity, the storage phase can be 300 seconds and more. However, it is usually between 60 and 120 seconds. The duration of the regeneration phase, on the other hand, is considerably shorter. It is less than 20 seconds.
It is customary for a nitrogen oxide sensor to be arranged downstream of the storage catalyst in order to determine the optimum instant for switching the engine from the storage phase to the regeneration phase. If the nitrogen oxide concentration in the exhaust gas measured by this sensor rises above a preset threshold value, regeneration of the catalyst is initiated.
Modern nitrogen oxide storage catalysts have a working range, based on the exhaust gas temperature upstream of the catalyst, of between approximately 150 and 500° C. This range is also referred to hereinbelow as the activity window. Below the activity window the storage catalyst cannot store the nitrogen oxides contained in the exhaust gas in the form of nitrates, since its catalytically active components are not yet able to oxidize the nitrogen oxides to form nitrogen dioxide. Above the activity window, the stored nitrates are thermally decomposed and released to the exhaust gas as nitrogen oxides. However, this “thermal desorption” does not occur suddenly above the activity window, but rather starts to occur, in competition with the storage process, even within the activity window.
The nitrogen oxide conversion which can be achieved with a nitrogen oxide storage catalyst, therefore, increases continuously as the bed temperature of the catalyst rises, passes through a maximum at medium temperatures within the activity window and then drops again at high bed temperatures. The position of the activity window and in particular the bed temperature for maximum nitrogen oxide conversion is dependent on the formulation of the catalyst, in particular on the nature of the storage components used. If alkaline-earth metal oxides, such as barium and strontium oxide, are used as storage components, the bed temperature for maximum nitrogen oxide conversion is approximately between 350 and 400° C.
During regeneration of a storage catalyst, there is a risk of the bed temperature of the catalyst during and after regeneration passing into a range in which the incipient thermal desorption considerably reduces the storage capacity of the catalyst on account of the heat which is released during the conversion of the nitrogen oxides by the reducing constituents of the exhaust gas. The inventors have observed that an accelerated release of unconverted nitrogen oxides can occur due to the relatively high temperatures. Furthermore, the inventors have observed that, in the case of specific operating parameters of the engine, the heating of the nitrogen oxide storage catalyst can extend through the regeneration and into the following lean-burn phase. As a result, there is brief slippage of nitrogen oxides through the catalyst, which can be so strong that the engine controller immediately initiates regeneration again.